1. Field of the Invention
The present invention relates to a driving circuit, especially to a light-emitting device driving circuit with an adaptive control, which implements a negative feedback circuit to control a tunable differential circuit for automatically compensating the output response changed by different operation conditions and obtaining an optimal modulation current output waveform.
2. Description of Related Art
FIG. 1a illustrates a light-emitting device driver including two cascaded differential stages. In FIG. 1a, the circuit 10 includes a differential gain stage and a differential output stage. As shown in FIG. 1a, the differential gain stage is formed by three FETs 16, 18, 20 and two load resistors 11, 12. The differential output stage is formed by a load resistor 13, three FETs 22, 24, 26 and a laser diode 14. In the differential gain stage, the gates of FETs 18, 20 are respectively connected to the outputs PA, PB of the previous differential gain stage, the sources are connected to the drain of FET 16, and the drains A, B are connected to a positive operating voltage source VDD through resistors 11, 12. Also, the drain A is connected to the gate of FET 26 and the drain B is connected to the gate of FET 24. Thus, a differential output voltage VDIFF with the polarity opposite to the front is generated for driving the differential output stage to output a current output. The gate of FET 16 is connected to a control voltage U to control the gain output of the differential gain stage and the source of FET 16 is connected to a negative operating voltage source Vss. In the differential output stage, the drain of FET 24 is connected to the positive operating voltage source VDD through resistor 13. The sources of FETs 24, 26 are connected to the drain of FET 22. The drain of FET 26 is connected to the positive operating voltage source VDD through a laser diode 14. The source of FET 22 is connected to the negative operating voltage source Vss, the gate C is used to receive a control voltage C so as to control the desired output current ILASER through the laser diode 14. The light output on the laser diode is controlled by the desired output current changed by the differential output voltage VDIFF. The curve of output current-differential voltage (I-V) is shown in FIG. 1b. In the curve CASE1, the current ILASER and the voltage VDIFF present a proportional relationship and the voltage VDIFF is a constant as controlled by the input gate control voltage (for example, U) . However, problems arise with this circuit when the circuit must operate in a relatively low modulation current (for example, in the range of 10-20 mA), as shown in CASE2. That is, the large value of VDIFF supplied as the input to FEDs 24, 26 of the output stage will overdrive these devices in the presence of the low current level supplied by FET 22. As a result, the laser output will overshoot and generate duty cycle distortion.
FIG. 2a illustrates another light-emitting device driver including two cascaded differential stages. In FIG. 2a, compared to FIG. 1a, the circuit is the same as that of FIG. 1a except for the gain output control and the output modulation control. As shown in FIG. 2a, the gain output control and the output modulation control are externally connected to a same control voltage Uxe2x80x2, different from different control voltages U and C, to control gates of FETs 50 and 56 and generate a dynamic gain control for the current output waveform. As such, as shown in FIG. 2b, the operating point positioned at either CASE1 or CASE2 can adjust the VDIFF operating range depending on the modulation current in the range of R so as to avoid the overshoot. However, for such a dynamic gain control circuit, when the operating temperature and/or the processes are changed, the operating point (condition) is changed so as to create problems. Unfortunately, the above-mentioned circuit cannot respond to the variance flexibly. For example, a duty cycle distortion is required to obtain the optimal current output characteristics.
Accordingly, an object of the invention is to provide a light-emitting device driving circuit with an adaptive control, which implements a negative feedback circuit to control a tunable differential circuit for automatically compensating the output response changed by different operation conditions and obtaining an optimal modulation current output waveform.
The invention is a light-emitting device driving circuit, which implements a negative feedback circuit to make the differential gain stage auto-adjusted to the optimal gain and thus to obtain the optimal current output waveform at any operating condition, so as to eliminate the overshoot and duty cycle distortion. The light-emitting device driving circuit includes: a tunable gain-controlled differential amplifier; a first differential output stage and a negative feedback control circuit. The negative feedback control circuit further includes: a detection circuit, a comparison circuit and a second differential output stage. The detection circuit acquires a current gain level and output the current gain level to the second differential output stage to generate an output current. The comparison circuit compares the output current and a predetermined reference current and feeds the comparison result back to the detection circuit and the tunable differential gain stage. Thus, auto-adjustment of the gain is achieved to optimize the output waveform of the first differential output stage.